As the auto-focus (AF) control of a video camera, etc., the TV-AF method is located in the mainstream thereof, which generates AF evaluation value signals expressing sharpness (contrast state) of video signals generated using image-pickup elements, and searches for the focus lens position where the AF evaluation value signals are maximum.
In addition, there is an external ranging method (that is, the external phase difference detection method) in the AF method, which has a ranging sensor provided independently from an image-pickup lens, calculates the in-focus position of the focus lens based on the distance to an object detected by the ranging sensor, and moves the focus lens to the in-focus position.
In the external phase difference detection method, a light flux received from the object is divided into two, and a pair of light-receiving element arrays (line sensors) receive the two-divided light fluxes, respectively. The method detects a shift amount between the images formed on the pair of line sensors, that is, a phase difference, obtains the object distance based on the phase difference by using triangulation, and moves the focus lens to an in-focus position corresponding to the object distance.
Further, there is a hybrid AF method having the above-described AF methods combined therein. In the hybrid AF method, for example, after the internal phase difference detection method drives the focus lens to the vicinity of an in-focus position, the method further accurately drives the focus lens to the in-focus position by the TV-AF method (see Japanese Patent Laid-Open No. 5-64056). Also, there is another hybrid AF method in which the external phase difference detection method and the TV-AF method are combined (see Japanese Patent Laid-Open No. 2005-234325). The hybrid AF method disclosed in Japanese Patent Laid-Open No. 2005-234325 selects a method using the focus control, of the TV-AF method and the external phase difference detection method, depending on change amounts of the respective signals in these methods.
The above-described hybrid AF method moves the focus lens to the vicinity of the in-focus position by the AF method other than the TV-AF method, and changes the AF method to the TV-AF method only when the AF evaluation value is higher than a predetermined level (that is, in a case of being close to the in-focus position) at this moment to move the focus lens to the in-focus position.
However, the AF evaluation value fluctuates in accordance with a change in the picture of the object regardless of a change in the object distance. In this case, if initially the focus lens is always moved by an AF method other than the TV-AF method, the focus will change even in an image-pickup condition in which the focus is not required to change, thereby generating unnecessary blur of the picked-up image. In particular, in an image-pickup apparatus for moving images in which the images are recorded during the AF operation, a blurred moving image will be recorded due to such unnecessary operations.
Also, when the external phase difference detection method is used as an AF method other than the TV-AF method, a parallax occurs between the image-pickup area of an image-pickup lens and the ranging field of a ranging sensor. If the parallax occurs, the focus lens is subjected to movement since the object distance detected by the external ranging sensor changes although an in-focus state is obtained in the TV-AF method. That is, unnecessary blur of the picked-up image occurs.
Further, the external phase difference detection method is not necessarily a highly reliable detection method of object distances when the objects include near one and far one, the object has an alternate pattern such as a stripe pattern, and the object has a low contrast. Also, in the external phase difference detection method, the detection accuracy of the object distances is reduced due to movement of the object and hand jiggling of a user.
Still further, although the determination conditions of reliability for detection of object distances can be tightened up, this takes much time for determination, which deteriorates high speed focusing performance that is a feature of the external phase difference detection method.